From the general state of the art, generic gearbox assemblies are known, in which the mechanical power transmission is combined with an infinitely variable hydrostatic gearbox. The purpose of this gearbox assembly is to enlarge the range of transmission of a hydrostatic drive and to make gear switching possible without interrupting the drive power. Typically a hydrostatic gearbox consisting of a hydraulic pump with variable speed and a downstream hydraulic motor, outputs a continually variable output speed to the wheels or tracks of the vehicle. In particular, the speed can be continually varied by controlling the pumping capacity of the variable hydraulic pump. In order to also cover an additional driving mode, the motor torque is divided between the hydrostatic gearbox and a mechanical gearbox coupled with it. The mechanical gearbox in the form of at least one planetary gear stage has a number of gearbox ranges corresponding to the various operating speeds of the wheel drive. The combination of the hydrostatic gearbox and the mechanical gearbox makes it possible to provide a continually variable gearbox which works over a wider range of speeds than would be possible with the use of the hydrostatic gearbox alone.
DE 101 44943 A1 discloses such a gearbox assembly. The gearbox assembly comprises a hydrostatic gearbox coupled with a mechanical gearbox so that first and second driving modes can be selected. A control apparatus carries out a switching operation from the first gearbox range provided for it to the second gearbox range. This state of the art solves the problem of not converting in a power-distributed way the whole of the drive power in the driving mode around the stationary position of the vehicle, so that the vehicle can be reversed in a purely hydrostatic way without having to switch a coupling in the stationary position, and that in this driving mode, which typically is the main working range for the vehicles, there is no reactive power present in the hydraulic cycle. The couplings should only be switched when the driving mode is changed and with the speed of the parts to be coupled being equal.
A drawback with this prior art is that the above result is only implemented with a great deal of structural complexity. A great number of individual couplings are necessary within the gearbox which must be driven in a coordinated way by a central electronic control apparatus.